Dual secondary signal transformer



Oct. 26, 1954 J' BARlNG 2,692,969

DUAL SECONDARY SIGNAL TRANSFORMER Filed May 11, 1950 2 Sheets-Sheet 1CURRENT REGULATING cmcun CURRENT REGULATING I cmcun CONTROL SERVOAMPLIFIER MOTOR JOHN A. BARING INVENTOR.

AT ORN EYS Oct. 26, 1954 A BARING 2,692,969

DUAL SECONDARY SIGNAL TRANSFORMER 2 Sheets-Sheet 2 Filed May 11, 1950 OO O O O O 5 4 3 PRESSURE-POUNDS/ SQUARE IN.

2o 40 so PRESSURE-POUNDS/SQUARE IN.

F I G. 8

JOHN ABARING IN ENTOR BY )0 I k ATT NFYS PRESSURE-POUNDS/SOUARE IN.

FIG. 9

Patented Oct. 26, 1954 2,692,969 DUAL SECONDARY SIGNAL TRANSFORMER JohnA. Baring, Evanston, Ill., assignor to Askania,

Regulator Company, Chicago, 111., a corporation of Illinois ApplicationMay 11, 1950, Serial No. 161,455

6 Claims.

The present invention relates to transformers of the type having asecondary winding system divided into two distinct parts or sections byan output terminal, and a core or" magnetically permeable materialinductively coupling such secondary system with a primary winding. Avery useful arrangement of such a transformer comprises provision forrelative movement between the core and secondary winding system, so thatdegree of coupling of the respective parts of the secondary system; andconsequently the relative amplitudes of voltages induced in such partsrespectively may be selected by appropriate posi tioning of the corerelative to such windings or parts. By such an arrangement theintermediate output terminal of the secondary system can be used as avoltage reference point, and between such point and the opposite ends ofthe secondary system can be obtained voltages of opposite phase senseand of amplitudes depending on the position of the core relative to thesecondary system parts. By applying such voltages to the opposite endsof an impedance provided with an intermediate tap, a signal voltage maybe developed between the respective intermediate taps of the secondarysystem and impedance. The arrangement presents several possibilities forsignal voltage development. The intermediate tap of the impedance may befixed, as at the electrical center point of the impedance, and thesignal voltage developed between it and the intermediate output terminalof the secondary system, which also is assumed to be a center tap, willcorrespond in phase sense to the direction of deflection of the corepiece from a neutral or zero position wherein the voltages induced inthe respective secondary system parts are of equal amplitudes, theamplitude of such signal voltage being proportional to degree of suchdeflection of the core piece from its neutral position. The intermediatetap of the impedance may be variable, and adjustable to balance out thesignal by variation of the voltage point of the impedance to produce atap voltage equal in amplitude to that of the secondary output terminal,again assumed to be a center tap. In the latter type of arrangement theadjustment of the variable tap, or the ratio of impedance values lyingto opposite sides of it when its output voltage balances that of thesecondary tap, provides a measure of the position of the magneticallypermeable core piece.

These variable dual secondary transformers have proven extremelyvaluable for use as signal sources for translation of mechanical signalsto corresponding alternating signal voltages. Such mechanical signalsare applied to move the core piece to a distance from a selected Zeroposition corresponding to signal magnitude. Connection of the secondarysystem to an impedance having a variable intermediate tap, in the mannerdescribed above, provides a bridge type signal voltage system the outputof which, taken across the respective intermediate tape of the secondarysystem and impedance, may eifectively be applied to a null circuitconstituting the signal input to a control amplifier that is selectivelyresponsive to alternating voltage of either phase sense. Such controlamplifier may be used to drive a servomotor, and if such motor beconnected to drive the variable tap of the impedance in usualsignal-canceling arrangement to proportion motor operation to magnitudeof unbalance of the bridge, an effective motor position control isestablished.

There are commercially available signal transformers of the dualsecondary and movable core type that are very satisfactory insofar asresponse of secondary voltage amplitudes to core piece position changealone is concerned, such transformers having very nearly linearcharacteristics of response of variation of secondary voltage withchange of core piece position. These transformers, however, are subjectto appearance of disturbance voltages that appear when the voltagesinduced in the respective parts of the secondary system are of equalamplitudes, and that also appear in the bridge type arrangement provided with a variable impedance having an intermediate tap, when suchtap is adjusted so that its voltage amplitude equals the voltage amplitude at the secondary output terminal, which should result in completebalancing out of voltage between such tap and. terminal. These voltages,hereinafter termed null voltages because of their appearance inconditions of theoretical voltage balance, appear substantially inquadrature phase relation to the voltage across the entire secondarysystem, or resultant of the respective secondary voltages on oppositesides of the secondary intermediate output terminal. They appear atminimum amplitude in the neutral position of the core piece, whereinvoltages of equal amplitudes are induced in the respective parts orsections of the secondary winding system, and increase in somewhatirregular functional relation with deflection of the core piece fromthat position in either direction, and accompanying variation inrelative amplitudes of the voltages induced in the respective parts ofthe secondary system.

Null voltage characteristics of dual secondary transformers, andespecially dual secondary transformers having movable core pieces forvariation of secondary output voltage amplitude, heretofore have undulylimited the types of service of such transformers. If such a transformeris used as a source of signal voltage for a direct-current amplifier,the null voltage constitutes a false signal if its amplitude range is inthe response sensitivity range of the amplifier. While false signaleffects of null voltage can be eliminated by employing a properly phasedalternating current amplifier, such voltage tends to load or saturatethe amplifier, seriously impairing its sensitivity, and in certain casesserving to restrict the useful range of transformer output voltage to avery limited region in the immediate neighborhood of the neutralposition of the transformer core piece and wherein the null voltageamplitude is within the saturation tolerance limit of the amplifier. Thepresent invention is devoted to the problem of reducing null voltagesubstantially and at least to a level that will materially extend thefields in which transformers of the type in question are practicallyuseful.

Null voltage, which as stated appears in quadrature phase relation withthe total voltage across the entire secondary system of a dual secondarytransformer, appears to arise from, or at least to accompany, variancefrom phase agreement between the respective secondary voltages, suchphase disagreement apparently being of minimum degree when theamplitudes of the respective secondary voltages are equal, andincreasing with movement of the core piece from the position productiveof such equal voltage amplitudes. It is evident that such core piecedeflection is accompanied by change in the degree of coupling betweenthe respective secondary windings and the primary winding. The presenceof null voltage at the neutral position of the core piece, wherein theamplitudes of the respective secondary voltages are equal, may arisefrom either or both an actual inequality of coupling of the respectivesecondary windings with the primary winding when the core piece has beenadjusted to a position productive of equal secondary voltage amplitudes,possibly arising from unavoidable differences in the electricalcharacteristics of the respective secondary windings, and/ornon-uniformity of the physical structure of the core piece resulting innon-uniform magnetic characteristics and in variance between intensitiesof magnetic field portions interlinked with the two windings in closerand more remote degrees. Whatever may be the true cause of null voltage,a corrective measure applied to the respective secondary windings ingeneral proportional relation to the closeness of their respectivecouplings with the primary winding, or to the respective amplitudes ofvoltages induced in them when the core piece is out of its neutralposition, and of a character tending to shift the respective secondaryvoltages toward a common phase, results in a marked reduction of nullvoltage, both at the neutral core piece position and throughout therange of core piece position variation, as will be described fully inthe following disclosure.

A primary object of the invention is the provision in a dual secondartransformer having a magnetically permeable core piece so positionedrelative to the respective secondary windings as to induce in themvoltages of equal amplitudes, of a novel arrangement for reducingsubstantially the amplitude of null voltage appearing substantially inquadrature phase relation with the voltage appearing across thesecondary system.

Another primary object of the invention is the provision in a dualsecondary transformer having a movable core piece, of novel means forreducing throughout the range of variation of such core piece thecorresponding amplitude range of such null voltage.

Another object is the provision in a dual secondary transformer, ofnovel means for reducing phase disagreement between voltages induced inthe respective secondary windings.

Another object is the provision in a dual secondary transformer that isprovided with a movable core piece for varying inversely the voltagesinduced in the respective secondaries, of novel means for substantiallyreducing adverse effects on the output of the secondary systemaccompanying movement of the core piece.

Still another object is the provision of a novel arrangement of centertapped secondary, variable transformer, capable of highly satisfactoryuse in a null circuit bridge arrangement with a signal cancelingimpedance connected across the transformer secondary system and having avariable intermediate tap providing Voltage for comparison with thesecondary center tap voltage for development of an error signal.

A further object is the provision of a dual secondary transformer, withnovel means for applying to the respective secondary windings correctivevoltages the effects of which on the respective secondary voltagesinduced from primary current automatically are proportioned to adverseeffects on such voltages accompanying different degrees of closeness ofcoupling of the secondary windings with the primary winding.

In the accompanying drawings:

Fig. 1 is a schematic diagram illustrating a basic and broad aspect ofthe invention, disclosing its application to a dual secondarytransformer having a core piece disposed for induction of voltages ofequal amplitudes in the respective secondary windings.

Fig. 2 is an exaggerated vector diagram disclosing a theory ofproduction of null voltage in an arrangement such as that of Fig. 1, andalso illustrating the mode of performance of the invention, and a theoryof its mode operation.

Fig. 3 is a schematic diagram disclosing application of the invention toa variable dual secondary transformer used as a source of variable andreversible phase sense alternating voltage in a null circuit bridgearrangement constituting the signal circuit of a servomotor positioncontrol system.

Fig. 4 is a vector diagram similar to Fig. 2 and illustrating the methodof performing the invention and a theoretical mode of its operation, asapplied to a variable dual secondary transformer.

Fig. 5 is a schematic median section of a variable dual secondarytransformer embodying a form of the invention.

Fig. 6 is a schematic diagram disclosing use of the invention in asystem for measuring pres sure magnitude by means of a variable dualsecondary transformer.

Fig. 7 is a graphical diagram disclosing the sensitivites of the sametransformer, arranged in the system of Fig. 6, respectively with andwith out employment of the invention.

Figs. 8 and a are graphs, both greatly enlarged with respect to thescale of Fig. 7, and further respectively enlarged vertically by greatlydifferent factors, disclosing null voltage ranges of the sametransformer, connected as in Fig. 6, and respectively without and withapplication of the invention.

Describing the drawings in detail and first referring to Fig. 1, atransformer is disclosed as having a primary winding 1 0, a pair ofsecondary windings H, I 2 and a core iii of magnetically permeablematerials inductively coupling the respective secondary windings to theprimary winding. It is assumed that the relative positional relationsbetween the respective secondary windings and the core piece are suchthat voltages of equal amplitudes are induced in the respectivesecondary windings by current flowing in the primary winding iii. Thewindings H, 12 are shown as connected in series aiding relation, and anoutput terminal or center tap I4 is connected between them to serve as avoltage reference point. The electrically opposite ends of therespective secondary windings H, 12, that is to say the opposite ends ofthe secondary winding system, are shown connected to opposite ends of acenter tapped resistance i5.

This arrangement, which is shown principally for purposes of disclosureand explanation of one aspect of the invention, is such that no voltageshould appear across terminals 56, I7, respectively connected with thesecondary center tap l4 and the center tap of resistance !5, since bothconstitute midpoints between voltages that are equal and opposite.Actually, however, a voltage does appear across these terminals, suchbeing the null voltage referred to above, and appearing in phasequadrature relation to the resultant voltage of the two secondarywindings, appearing across the entire secondary winding system.

Referring now to Fig. 3, the same reference numerals are applied to thetransformer windings as in Fig. 1. In Fig. 3 the magnetically permeablecore comprises a core piece 20, movable by a rod 2! in directionsparallel to the axes of the various windings for varying the closenessof coupling of the respective secondary windings i i, i2, with primarywinding i0, thereby to select the amplitudes of voltages induced in themrespectively by current flowing in the primary winding. In Fig. 3, theterminal H is connected to the variable contact or tap 22 of apotentiometer, the resistance 23 of which is connected between theelectrically opposite ends of the respective secondary windings it, 52,in other words across the secondary system or" the transformer. In thistype of arrangement, with the core piece displaced from its neutralposition it is theoretically possible to move the potentiometer tap 22to a corresponding position wherein no voltage will appear acrossterminals it, it it being possible to adjust the position of contact 22to a point on resistance 23 at which no potential exists hetween suchtap and center tap it of the second ary winding system. Actually, eventhough the potentiometer tap 22 is moved to such a position that thevoltages across the parts of resistance are respectively equal inmagnitude to the voltages induced in the respective secondary windings,a null voltage will appear across terminals it, ill. Such voltage, asstated in the preliminary part of this specification, varies in somewhatirregular functional relation with displacement of the .core piece fromits neutral position.

The vector diagrams, Figs. 2 and 4, respectively disclose a possibleexplanation of null voltage and the functional relation that itsamplitude bears to core piece position change. These vector diagramsalso illustrate the manner in which null voltage-reducing or correctivevoltages are applied to the respective secondary windings, and apossible explanation of their efiectiveness in reduction of nullvoltage.

Referring first to Fig. 2, vector a represents the voltage induced byprimary current in one of the secondary windings, as H, of Fig. 1, andvector 1) represents the voltage induced in the other secondary winding.The appearance of a null voltage across terminals I6, I l reasonably isexplained as the result of the indicated phase lag by vector b of vectora, resulting in the appearance of a quadrature voltage represented byvector 0, and that is of amplitude proportional to both amplitudes ofthe respective secondary voltages and degree of phase displacementbetween them. Actually, the phase lag and magnitude of quadrature nullvoltage are greatly exaggerated in the vector diagrams Figs. 2 and 4.The resultant of the two secondary voltages, or the total voltageappearing across th secondry winding system is represented by the dottedvector at.

In its broadest aspect, the invention resides in applying to therespective secondary windings corrective voltages of a common phase, butof different amplitudes, each having an out of phase relation to thesecondary voltage induced from primary current in the secondary windingsto which it is applied and therefore tending to shift the lattervoltages in the same direction but to different degrees as determined bysuch voltage amplitudes. The theory of this procedure is that theamplitudes of such corrective voltages are roughly proportioned todegree of displacement of the respective secondary voltages from acommon phase and therefore have a tendency to shift such voltagesdifferent degrees and into much closer phase correspondence.

As a means of generally proportioning the relative amplitudes ofcorrective voltages to the different null voltage-producing effects thatact in the respective secondaries, and based on the theory that sucheffects arise from or accompany different degrees of closeness ofcoupling of the respective secondaries with the primary winding, andtherefore are functionally related to relative number of interlinlragesbetween the magnetic field of the core and the turns of the respectivesecondary windings, the corrective voltages conveniently are applied tothe secondaries by in ducing them in the secondary windings throughinterlinlrages with the respective secondary windings of an auxiliarymagnetic field developed at the core, the numbers of such interlinkageswith the respective secondaries having similar proportional relation tothat existing between the respective secondary windings and the corefield generated by primary current.

Referring again to Fig. i, it will be seen that an auxiliary primarywinding 25 i coupled with the secondary windings ii, through core it,such winding being shown as encircling the core so that upon energizingauxiliary winding 25 by alternating current, auxiliary voltages therespective secondary windings ii, are induced from such current. It willbe seen that if the auxiliary voltages can be made to have on theprincipal secondary voltages corrective eifects that tend to counteractwhatever dif-= ference in the characteristics of the two windings isresponsible for null voltage, their application should. result in nullvoltage reduction. It has been found that proper phasing, with respectto secondary voltage, of alternating current used to energize anauxiliary primary winding arranged as in Fig. 1, actually results in amarked reduction of null voltage.

A corrective eiiect is given the auxiliary voltages induced in therespective secondaries by so phasing such voltages relative to theprincipal secondary voltages that the latter tend to be shifted in phasein a common direction, and it is my theory that since the auxiliaryvoltages are induced in the respective secondaries throughinterlinirages of the latter with the auxiliary core held, the degree ofphase shift or the respective secondary voltages accomplished by theauxiliary voltages is related to the unequal interlinkages or therespective secondaries with the main core field, or with relativecloseness of the respective secondaries with the primary that areresponsible for, or accompany null voltage production.

Again referring to Fig. 2 it will be seen that if the phases of thesecondary voltages respectively represented by vectors a and b can beadvanced respectively by degrees represented by angles e and er, therespective voltages can be brought into phase agreement, as representedby vector I, so that vectors representing them are colinear with eachother and therefore with their resultant g, and no quadrature or nullvoltage appears. Corrective voltages for accomplishing these phaseshifts are represented by vectors h and i, such voltages being in phaseagreement and leading the resultant of the principal secondary voltagesby a substantial angle, as indicated by the angular position of thevectors that represent them. Empirical experiment has indicated that aslight leading phase angle of current used to energize the auxiliaryprimary winding, with respect to phase of the voltage across thesecondary system, is productive of the best null voltage reductionefiect. Actually, and regardless of theory of mode of itsaccomplishment, the provision of an auxiliary primary winding arrangedas in Fig. l and energized by a current having a phase relation tovoltage across the secondary system of a dual secondary transformer of aslightly leading character, somewhat as represented by the relation ofvector k of Fig. 2 to vector g, results in a null voltage reduction. ofapproximately eighty-eight percent, as will later be described indetail.

Production, and correct phasing of the current that energizes theauxiliary primary winding, conveniently and practically can beaccomplished by developing it from voltage appearing across thesecondary winding system. As shown in Fig. 1, the winding 25 may becoupled parallel to the secondary winding system, by a circuit havingits input terminals 26, 2? connected to the opposite sides of thesecondary system. For proper phasing of the current energizing winding25 relative to the voltage appearing across the secondary winding systemM, 2, and for regulating amplitude of such current, a current regulatingsystem 28 may be connected in the circuit with winding 25, and mayinclude any conventional phase and amplitude selective arrangements.

In Fig. 3, an auxiliary primary winding 38 is shown mounted on, andmovable with the movable core piece 29. This auxiliary primary winding38 is energized by voltage developed across the entire secondary systemor" the transformer,

and the phase and amplitude of current supplied to it is selected by acurrent regulating system 38. It has been found that an arrangement ofthis lrind results in null voltage reduction, throughout the entirerange of core piece positlon variation wherein variation .of secondaryvoltage amplitude is linear, of approximately the same degree asmentioned above in the neutral condition of transformers of the kind inquestion, namely a reduction of approximately eightyeight percent ascompared to the null voltage range of the same transformers when notprovided with compensating or correcting auxiliary primary windings.

Fig. 4, which is quite similar to Fig. 3, represents voltage relationsin a variable dual secondary transformer in an unbalanced conditionresulting from deflection of its core piece from its neutral position.In such variable transformers, the null voltage characteristic, asstated above, has an irregular functional relation to degree of corepiece deflection. This is partly explainable as being the result ofincreasing variance between the inductances of the respective secondarywindings as their relative interlinkage with the core field are altered,and is advanced as supporting the theory that in the neutral conditionof a dual secondary transformer null voltage results from actuallydifferent degrees of such interlinkages of the respective secondarieswith the core field, and a resulting variance in their inductances and aphase disagreement between their voltages. It is believed that theincreased null voltage that appears when the core piece is displacedfrom neutral position accompanies increased phase disagreement betweenthe secondary voltages. Such an increased phase disagreement isrepresented by an angle between vectors a and b, which again representthe respective secondary voltages, as compared with the correspondingangle of Fig. 1. It will be seen that this increased phase angle resultsin a marked increase in the difference between angles 6 and :31 thatseparate the respective vectors a, b from some common angular position,such as that of vector y and representing phase agreement of thesecondary voltages, and consequently in a greater differential inamplitudes of corrective voltages that are necessary to advance the twosecondary voltages to such a phase agreement relation. However, it is tobe noted that the greater lag of vector (1 by vector b represents theeffect on the corresponding voltages of the same core piece deflectionthat also relatively increases the amplitude of voltage 17, while thecorresponding decrease in coupling of the other secondary responsiblefor decreased amplitude of its voltage may result in a decrease of itslag from a common leading phase. Consequently, since the corrective orphase advancing auxiliary voltages are induced in the respectivesecondaries through similarly unequal couplings that are responsible forthe relative phase variation between the voltages of the twosecondaries, and since amplitudes of such auxiliary voltages and theirrespective phase-advancing effects are related to such unequalcouplings, the phase advancing sheets of the auxiliary voltages on theprincipal secondary voltages automatically are related to the respectiveangle through which such voltages must be shifted to reach phaseagreement, and the corrective effects applied to the two secondariesautomatically are related to the need for them throughout the variationrange of the transformer.

Thus in Fig. 4, the amplitude of the corrective voltage represented byvector h, which is assumed to be induced in the secondary that producesvoltage a, is reduced as compared to the corresponding voltage of Fig.2, .and the amplitude of voltage 1' is correspondingly increased, theseamplitude relations accompanying the differential in interlinkage withthe auxiliary core field of the secondaries respectively producingvoltages a and b. agreement suggested by Fig. 1 between the two shiftedsecondary voltages, in Fig. 4 vectors a and b respectively have beenshifted by degrees of such magnitude that the vectors are still out ofphase but wherein they are in much closer phase than in their originalrelation. Existence of this condition may explain the residual nullvoltage, varying with transformer adjustment, that occurs in thecorrected system, as will appear later. The relation of the voltages,however, represents a marked improvement over the voltages of anuncorrected system.

In Fig. 3, an error signal voltage null circuit 32 is disclosed asconnected across the variable potentiometer tap 22 and the secondarycenter tap M. This null circuit 32.is connected as the control signalcircuit of a control amplifier 33 arranged to operate a servo motor 34in a direction to correspond to the sense of unbalance of the nullcircuit. A mechanical feedback or signal cancelling drive train 35 isarranged to move variable tap 22 toward a position corresponding to thatof the movable transformer core piece 2i during such direction-responseof the motor 34, thus constituting a well known null circuit bridge typeof servo motor position control system. The contribution of theinvention to this type of system will be pointed out later.

Fig. 5 discloses a simple arrangement for mounting an auxiliary primarywinding on the movable core piece of a standard variable dual secondarytransformer, for praticing the invention. The auxiliary winding 40 issupported directly on and surrounding the core piece 4|, and

Instead of the perfect phase is secured thereon by suitable means suchas adhesive or adhesive tape. The lead wires 42 for energizing thewinding may similarly be adhesively secured to the rod 43 that supportsand moves core piece ll, being shown as secured to such rod by tapes 44.A standard winding arrangement of such transformers is shown, comprisingin coaxial disposition a central primary winding 45 and a pair ofsecondary windings 46 on opposite sides of such primary winding. Corepiece ll is movable axially of the three windings in a central bore orpassage 41.

Figs. 6 and '7 to 9 respectively show an actual test setup of a typicaldual secondary variable transformer, and comparative graphs of thesensitivity and null voltage characteristics of the same transformerarranged in the test setup of Fig. 6 and with and without provision ofsuch transformer with the auxiliary primary winding and system forenergizing it described above with relation to Figs. 3 and 5.

In the actual test setup of the system of Fig. 6, the transformer was acommercially produced variable dual secondary transformer having alinear output voltage range of zero to eight hundred eighty millivolts,and the core piece 49 was operatively connected to the mechanical outputmember of a commercially produced Bourdon tube 5b having a linearresponse of movement of its output member to variations in appliedpressure through a range of zero to fifty pounds per square inch. Thecore piece and Bourdon tube were so positioned relative to the windingsthat at zero applied pressure the core piece occupied a position spacedto one side of its zero position to a degree rendering secondary voltagea linear measure of deflection of the core piece 49 from such zeroposition in the range of movement of the core piece by the full pressurerange of the Bourdon tube. The operating and supporting stem 5| of thecore piece was connected with the output element of the Bourdon tube 50.A vacuum tube voltmeter was connected across the center tap 52 of thetransformer secondary winding system 53, 54 and the variable tap 55 of apotentiometer, the resistance 55 of which was connected across thesecondary system of the transformer. For testing, the tap 55 wasadjusted manually for balance of the null circuit between the taps 52,55. Null voltages were measured by adjusting the potentiometer tap 55 tothe minimum obtainable voltage between the taps 52, 55, at variousmagnitudes of pressure applied to the input of the Bourdon tube 50.

For reduction of null voltage throughout the range of variation of thetransformer, an auxiliary primary winding 51 was mounted on the corepiece 49, in the manner disclosed by Fig. 5. The current regulatingsystem connected in series with auxiliary primary winding 51 included aseries connected current-limiting resistance 58, and a series connectedphase-adjusting network 59 also connected in series with the winding 57and including in parallel connection a resistance 60 and a condenser 6|.

Actual element values of the circuit elements. which were selected byempirical experiment giving optimum null voltage reduction, were asfollows. Current-limiting resistance 58 had a resistance of seventeenhundred fifty ohms. Resistance 60 had a value of eight hundred eightyohms, and condenser iii a capacity of one and six-tenths microfarads.Auxiliary primary winding 5'! was composed of five turns of numberthirty-six copper wire. Impedance of the entire circuit was twenty-fourhundred and fifty ohms at an angle of minus seven and six-tenthsdegrees, which was found to represent a desirable lead of the secondaryvoltage by the current energizing the auxiliary primary winding 51 fornull voltage reduction.

The graph m of Fig. 7 represents sensitivity of the transformer withoutthe corrective circuit, while graph 12 represents sensitivity of thesame transformer when corrected by the circuit arranged as disclosed byFig. 6 and as described above. Graphs m, n were obtained by plotting asabscissae pressures exerted in the Bourdon tube and as ordinates voltagebetween the intermediate secondary tap 52 and the variable potentiometertap 55 with the latter set a zero. The use of the null voltage-reducingcircuit resulted in a sensitivity loss, evidenced by the lower ordinatevalues of graph n of approximately five percent.

The reduction of null voltage amplitudes accomplished by use of thecorrective circuit disclosed by Fig. 1 and described above, isrepresented by the graphs of Figs. 8 and 9, which respectively representthe null voltage characteristic of the same transformer when uncorrectedand when corrected by the circuit in question. It is emphasized that thevertical scale of the null voltage amplitude characteristic of thecorrected transformer, shown by Fig. 9, is ten times greater aeeaeee ill than that of the null voltage characteristic of the uncorrectedtransformer, disclosed by Fig. 8. Thus it will be seen that in theuncorrected transformer, null voltage increased progressively, insomewhat irregular and asymmetrical fashion from a minimum amplitude ofapproximately one andone-half millivolts, occurring at the neutral corepiece position at the center of the pressure range, to maxima ofapproximately twelve and one-half millivolts, occurring at the end ofthe linear voltage response to core piece position variations,represented respectively by the extremes of the pressure range. As shownin Fig. 9, when corrected by the null voltage-reducing circuit inquestion, null voltage varied, again irregularly and asymmetrically,from a minimum of approximately seven-tenth millivolt, occuring at theneutral position to maxima of approximately one and four-tenthsmillivolts occurring at the respective ends of the range piece positionvariation. Thus a null voltage reduction of some eighty to eighty-fivepercent with a sensitivity loss of only five percent may be accomplishedby employment of the invention herein disclosed.

It has been observed during the course of extensive experiment relativeto null voltage in dual secondary transformers that loading thesecondary system results in a reduction of null voltage, but suchreduction is accompanied by a reduction in sensitivity to a degree notnearly ofiset by the null voltage reduction. It will be evident,therefore, that the resistance 56 of the potentiometer must besufficiently great to avoid loading the secondary. Also loading of thesecondary by the auxiliary primary circuit is to be avoided, by choosingresistor 58 of suitably high value. Potentiometer resistances of theorder of thirty to forty-five kilohms have been found to be satisfactoryfor a transformer having the output voltage range mentioned above. Itmay be noted here that inductive energization of the auxiliary primarywinding by inductive coupling with the primary winding is so negligible,as compared with its direct conductive energization by the circuitconnected across the secondary sys tem as to produce no appreciableeffect on the secondary voltages.

From the "foregoing the fundmental aspects and types of arrangement thatmay be made to practice the invention will be evident, and it will beunderstood that many variations may be made from the specificdisclosures without departing from the invention as defined by theappended claims.

I claim:

1. In a variable transformer that includes a main primary winding, apair of secondary windings, and a core piece of magnetically permeablematerial inductively coupling said windings and movable relative to themfor varying the respective degrees of coupling between the respectivesecondary windings and the primary winding; corrective means comprisingan auxiliary primary winding mounted on the core piece and movable withit, and circuit means for energizing said auxiliary primary winding withalternating current having a preselected out of phase relation withvoltage induced in the secondary windings.

2. In a variable transformer that includes a main primary winding, apair of secondary windings connected in series aiding relation, and acore piece of magnetically permeable material inductively coupling saidwindings and movable relative to them for varying the respective degreesof its couplings with the respective secondary windings, correctivemeans comprising an auxiliary primary winding mounted on said core piecefor movement with it, and circuit means for energizing said auxiliaryprimary winding, said circuit means having an input connected acrossboth said secondary windings, and including phase shifting means fordisplacing current energizing said auxiliary primary winding to apreselected degree relative to voltage across said secondary windingsinduced by current energizing said primary winding.

3. In a transformer that includes a main primary winding, a secondarysystem of multiple windings connected in series, and a core ofmagnetically permeable material coupling said windings; a correctivecircuit including an auxiliary primary winding coupled with saidsecondary windings through said core, an input coupled with saidsecondary system for energization by current induced in the latterduring energization of said main primary winding, and phase shiftingmeans connected in said circuit for producing a preselected phasediiierence between current energizing said auxiliary winding and voltageacross said secondary system induced by alternating current energizingsaid main primary winding.

4. A corrective arrangement for a variable transformer having a mainprimary winding, 2. pair of secondary windings connected in series, acore piece of magnetically permeable material coupling said secondaryand primary windings and means for varying the respective degrees ofcoupling of the different secondary windings with the primary winding,said arrangement comprising a circuit having output means inductivelycoupled with the respective secondary windings, means for increasinganddecreasing the coupling of said output means with the differentsecondary windings as the degrees of coupling of the same secondarywindings with the main primary winding are increased and decreased, andmeans for energizing said output means with alternating current offrequency corresponding to that of voltage across said secondary systeminduced by alternating current energizing said primary winding andhaving a preselected phase difference from the latter voltage.

5. In a variable transformer that includes a main primary winding and apair of secondary windings connected in series; an auxiliary primarywinding, magnetically permeable core means coupling both said main andauxiliary primary windings with said secondary windings and said coremeans being movable relative to said secondary windings for varying thedegrees of their respective couplings with both said primary windings,and circuit means arranged to energize said auxiliary primary windingwith alternating current of frequency corresponding to that of voltageacross said secondary windings and having a preselected degree of phasedisplacement from said voltage.

6. In a null circuit error signal bridge arrangement that includes avariable signal transformer having a main primary winding, dualsecondary windings connected in series aiding relation and with amidta-p connected between them, and a core piece of magneticallypermeable material coupling said primary and secondary windings andmovable relative to the latter; and a potentiometer having itsresistance connected across said secondary windings, and a variableintermediate tap; an auxiliary primary winding in- 13 ductively coupledwith said secondary windings by said core piece, and a circuit forenergizing said auxiliary primary windings, said circuit having an inputconnected across said secondary windings and including phase-shiftingmeans for displacing the phase of current energizing said auxiliarywinding to a preselected degree relative to voltage across saidsecondary windings induced by current energizing said main primarywinding. v

References Cited in the file of this patent Number UNITED STATES PATENTSName Date Harrison Sept, 1, 1936 Zeitlin June 22, 1943 Mauerer Sept. 19,1944 Hornfeck Apr. 20, 1948 Greenough Dec. 7, 1948 Fuller Sept. 27, 1949

